a. Field of Invention
The present invention relates to a plunging type tripod constant velocity universal joint for use in transmission of power in automobiles, industrial machines, and the like.
b. Prior Art
A conventional tripod constant velocity universal joint, as shown in the FIG. 4A, comprises an outer joint member 10′ having three axial track grooves 12′ in the inner periphery thereof and formed with roller guide surfaces 14′ in the opposed side walls of each track groove 12′, a tripod member 20′ having a trunnion barrel 21′ adapted to be fitted to a shaft for torque transmission and trunnion journals 22′ radially projecting from three circumferentially equispaced positions on the trunnion barrel 21′, and rollers 30′ each rotatable around the trunnion journal 22′ through a plurality of needle rollers 32′ and received in the track groves 12′ of the outer joint member 10′, the roller 30′ being guided in the outer peripheral surface by the roller guide surfaces 14′.
There are two forms of contact between the roller 30′ and the roller guide surfaces 14′; angular contact and circular contact. Angular contact has a certain contact angle and occurs at two points (FIG. 5A). Circular contact occurs at one point and the contact ratio generally employed ranges from 1.002 to 1.008 (FIG. 5B).
In the case of angular contact, when a contact ellipse is produced at two points in the direction of the contact angle and a predetermined torque T is applied, it is necessary that the contact ellipse be in the width of the roller 30′. For this reason, in the existing circumstance, the proportion of the width of the roller 30′ to the outer diameter ranges from 32% to 36%. Further, even if the contact angle and contact ratio are reconsidered, the total widthwise contact length exceeds the width of the roller 30′. The phenomenon of both ends of the roller 30′ cutting into the roller guide surfaces 14′ or the phenomenon of two contact ellipses overlapping each other in the middle of the roller 30′, which has been an obstacle to improvement of life and to the reduction of vibration, cannot be avoided.
In the case of conventional circular contact, since the contact ratio ranges from 1.002 to 1.008 when a predetermined torque T is applied, the widthwise contact ellipse length some times exceeds the width of the roller 30′. For this reason, there is a limit to the reduction of the width of the roller 30′ as in the case of angular contact. In the existing circumstances, the proportion of the width of the roller 30′ to the outer diameter ranges from 32% to 36%. Further, if the width of the roller 30′ is reduced, the total widthwise contact length far exceeds the width of the roller 30′, obstructing the improvement of life and the reduction of vibration.
Further, in both angular contact and circular contact, the roller guide surfaces 14′ have a radius of curvature R at a certain contact ratio and the major and minor inner diameters are, as such, connected. When the tripod joint is rotating while taking an operating angle, an angular displacement also occurs between the roller 30′ and the roller guide surfaces 14′. This causes wearing of the roller guide surfaces to proceed. Then, there occurs on both the major and minor diameter sides the phenomenon of both ends of the roller 30′ cutting into the roller guide surfaces 14′, forming a cause of increasing the vibration.
Accordingly, an object of the invention is to solve the problems as described, so as to achieve reduced vibration and hence weight reduction and compactification of a tripod joint without aggravating the vibration characteristics.
In the conventional tripod joint, the clearances (radial clearance, circumferential clearance) between the trunnion journal 22′, the needle rollers 32′ and the roller 30′ have not been set in consideration of the skew angle. The skew of a roller, as shown in FIGS. 6A, 6B and 6C, means that the axis of the roller inclines at a certain small angle (θ) rather than forming right angles with the direction of travel.
It has been found that the noise, vibration, hardness (hereinafter “NVH”) characteristic of tripod joints depends on the angle at which needle rollers can actually skew. The skew angle is determined by the radial clearance and circumferential clearance but this has not heretofore been taken into consideration. Therefore, the NVH characteristic differs according to differences in the proportions or size of a tripod joint and optimization of this situation has not been made at present.
Another object of the invention is, therefore, to reduce vibration of tripod constant velocity universal joints by suppressing the skew of needle rollers.
Generally, tripod joints are designed with particular attention paid to the aspect of strength (torsional strength). The strength is uniquely determined usually by the minimum outer diameter of the shaft, and then the strength of the tripod member or the strength of the roller is considered. The strength is evaluated mainly in two ways: static torsional strength (test), and pulsating fatigue strength (test). Usually, in the two tests, it is arranged that the shaft is the first to break. Therefore, it is arranged that the tripod member or the roller has a strength not less than that of the shaft.
The static torsional strength is evaluated, in a static torsional test with torque applied to a tripod joint, by a torque with which one region or another is torsionally broken. The pulsating fatigue strength is evaluated by the number of cycles with which one region or another breaks when a predetermined pulsating torque is applied to a tripod joint.
The strength of the tripod member depends on the strength of the root of the trunnion journal and the strength of the trunnion barrel. Increasing the trunnion journal diameter increases the strength of the root of the trunnion journal, and increasing the outer diameter of the trunnion barrel increases the strength of the trunnion barrel.
However, increasing the trunnion journal diameter would necessarily increase the outer diameter of the roller, while increasing the outer diameter of the trunnion barrel would involve an increase in the minor inner diameter of the outer joint member. Therefore, in achieving the weight reduction and compactification of the tripod joint, it is not sufficient simply to reduce the size of the shell of the outer joint member, since limitation is imposed on the operating region (geometry) of the tripod joint; therefore, balanced design of the various portions becomes important.
Further, rolling fatigue life (flaking life) of the rolling section, particularly between the needle rollers and the trunnion journal also has to be considered. In durability tests, when the tripod joint is driven at a predetermined rpm while applying a predetermined torque, the evaluation of durability is made by the number of cycles or time taken for flaking to occur. Usually, this durability can be improved by increasing the outer diameter or length of needle rollers or their number, as is known; however, this would lead to an increase in the size of the shell of the outer joint member.
Therefore, with the balance between strength and durability in mind, the present invention is also intended to reduce the size of the shell of the outer joint member as much as possible so as to achieve weight reduction and compactification of a tripod joint.